Interleukin 11 (IL-11) is a cytokine that stimulates proliferation, maturation and functional activation of platelets. Recombinant human IL- 11 is approved and marketed as a treatment for thrombocytopenia in certain patients undergoing myelosuppressive cancer chemotherapy, the market for which is estimated to be $500 million. IL-11 has a short in vivo half-life and must be administered daily by subcutaneous injection to achieve optimal efficacy. The investigators propose to create modified IL-11 proteins that are equal, or superior, to natural IL- 11 at stimulating platelet formation in vivo, but which require less frequent dosing, on the order of once per week to once every third week. During Phase I the investigators identified sites in IL- 11 that can be modified without affecting the protein's in vitro bioactivity. During Phase II, sufficient quantities of the modified IL-11 proteins will be manufactured for testing of pharmacokinetics and efficacy in normal rodents and in rodent models of chemically induced thrombocytopenia. In the clinical setting, the improved properties of the novel IL-11 proteins will reduce the amount of IL-11 required per patient, improve patient compliance, and result in considerable cost savings to patients and healthcare providers. Additional potential benefits include improved drug efficacy, ease of use and improved patient quality of life. Information gained from these studies will aid in creating long-acting versions of structurally related cytokines and growth factors for use in treating cancer, infectious disease and hematopoietic disorders [unreadable] [unreadable]